1. Field of the Invention
The present invention relates to preceramic polymeric fibers, ceramic fibers prepared therefrom and methods for their preparation.
2. The Prior Art
Generally, in preparing a shaped ceramic article such as a fiber from a preceramic polymer by pyrolysis at elevated temperatures, it is necessary, prior to pyrolysis, to render the shaped article infusible. Otherwise, the shaped article will melt upon pyrolysis and thus the desired shape will be destroyed. The most common method of rendering the shaped article infusible has been an oxidation treatment. This method has the disadvantage of incorporating large amounts of oxygen in the resulting ceramic article. For example, standard grade Nicalon.RTM. ceramic fibers, prepared from polycarbosilanes by Nippon Carbon Company Ltd., Tokyo, Japan, normally contain about 10-15 weight percent oxygen. High oxygen content results in decreased thermal stability of the ceramic materials at elevated temperatures.
Ceramic materials prepared from polycarbosilanes are known in the art. Verbeek et al in German Application Publication No. 2,236,078, which is hereby incorporated by reference, prepared ceramic materials by firing a polycarbosilane prepared by the pyrolysis of monosilanes at elevated temperatures in an inert atmosphere. Linear high molecular weight polymers such as polyethylene oxide, polyisobutylene, polymethylmethacrylate, polyisoprene and polystyrene were reported to improve the fiber spinning characteristics of the polycarbosilanes. The polycarbosilane fibers were rendered infusible prior to pyrolysis by either thermal oxidation, sulfidation or hydrolysis treatment. The ceramic fibers were reported to contain between 0 and 30 weight percent of oxygen, but no details were given.
Yajima et al in U.S. Pat. Nos. 4,052,430 Oct. 4, 1977) and 4,100,233 (Jul. 11, 1978), which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes in an inert atmosphere or in a vacuum at an elevated temperature. The polycarbosilanes were prepared by thermally decomposing and polycondensing polysilanes. Polycarbosilane fibers were treated for 2-48 hours at 350.degree.-800.degree. C. under vacuum prior to pyrolysis to remove low molecular weight material. In some cases, the fibers were first exposed to an oxidizing atmosphere at 50.degree.-400.degree. C. to form an oxide layer on the fibers and then treated under vacuum at 350.degree.-800.degree. C. The oxygen content of the resulting ceramic fibers was not reported.
Yajima et al in U.S. Pat. Nos. 4,220,600 (Sep. 2, 1980) and 4,283,376 (Aug. 11, 1981), which are both hereby incorporated by reference, prepared ceramic materials by the pyrolysis of polycarbosilanes partly containing siloxane bonds at an elevated temperature under an inert atmosphere or a vacuum. These polycarbosilanes were prepared by heating polysilanes in the presence of about 0.01 to 15 weight percent of a polyborosiloxane in an inert atmosphere. Polycarbosilane fibers were rendered infusible prior to pyrolysis by either treatment with an oxidizing atmosphere at about 50.degree.-400.degree. C. to form an oxide layer on the fiber surface or by irradiation with gamma-rays or an electron beam under an oxidizing or non-oxidizing atmosphere. The oxygen content of the resulting ceramic fibers were in the range of 0.01 to 10 weight percent by chemical analysis. Oxygen in the form of silica could be further removed from the ceramic fiber by treatment in a hydrofluoric acid solution.
Iwai et al in U.S. Pat. No. 4,377,677 (Mar. 22, 1983), which is hereby incorporated by reference, also produced ceramic materials by the pyrolysis of polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The polycarbosilanes of Iwai et al were prepared by heating a polysilane at 50.degree.-600.degree. C. in an inert gas, distilling out a low molecular weight polycarbosilane fraction and then polymerizing the distilled fraction at 250.degree.-500.degree. C. in an inert atmosphere. Polycarbosilane fibers were rendered infusible prior to pyrolysis by heating at relatively low temperatures in air. The oxygen content of the resulting ceramic fibers was not reported.
Schilling et al in U.S. Pat. No. 4,414,403 (Nov. 8, 1983), which is hereby incorporated by reference, produced ceramic material by the pyrolysis of branched polycarbosilanes at elevated temperatures under an inert atmosphere or vacuum. The branched polycarbosilanes were prepared by reacting monosilanes with an active metal in an inert solvent at elevated temperatures where at least some of the monosilanes contained vinyl groups or halomethyl groups capable of forming branching during the polymerization. Methods of rendering the material infusible were not discussed.
Yajima et al, J. Mat. Sci., Vol. 13, p. 2569 (1978); Yajima, Bull. Amer. Ceram. Soc., Vol. 62, p. 893 (1983); and Hasegawa et al, J. Mat. Sci., Vol. 18, p. 3633 (1983) also discuss polycarbosilanes which are useful as preceramic polymers for preparing silicon carbide ceramics. In the Bull. Amer. Ceram. Soc. article, Yajima prepared ceramic fibers from polycarbosilanes which had been rendered infusible prior to pyrolysis by heating in air at 190.degree. C. The resulting fibers contained 15.5 weight percent oxygen, most of which was thought to be incorporated into the fiber during the curing step.
Baney et al in U.S. Pat. No. 4,737,552 Apr. 12, 1988), which is hereby incorporated by reference, relates to a method of rendering a preceramic polycarbosilane composition infusible prior to pyrolysis by treating the preceramic polycarbosilane composition at a temperature of 150.degree.-400.degree. C. under an inert atmosphere or vacuum for a time sufficient to render the preceramic polycarbosilane composition infusible wherein the preceramic polycarbosilane composition contains (1) a polycarbosilane, (2) a hydrosilylation catalyst, and (3) an unsaturated compound selected from the group consisting of reactive alkynes, polyolefins, vinylsiloxane and unsaturated siloxanes. Baney et al also disclose a method of rendering preceramic polycarbosilane polymers infusible prior to pyrolysis by treating the preceramic polycarbosilane composition with a gas selected from the group consisting of reactive diolefins, reactive alkynes and vinylsilanes at a temperature of 150.degree.-400.degree. C. for a time sufficient to render the preceramic polycarbosilane composition infusible wherein the preceramic polycarbosilane composition contains (1) a polycarbosilane and (2) a hydrosilylation catalyst.
Seyferth et al in U.S. Pat. No. 4,719,273 Jan. 12, 1988), the entire contents of which are incorporated herein by reference, describe a process of reacting a polycarbosilane with an organo-silicon compound having at least two alkenyl groups and then forming fibers from the reaction product.
Bujalski et al in U.S. Pat. No. 4,889,899 Dec. 26, 1989), the entire contents of which are incorporated herein by reference, describe a method of preparing fibers employing a vinylic polysilane.
The use of polycarbosilane (PC) as a precursor to SiC fibers was first reported by Yajima et al in Nature, Vol. 261, p. 683 (1976). This method involved the synthesis of PC from a polydimethylsilane precursor. The PC was then spun into fibers and reacted with oxygen in order to cross-link the fibers and keep them from melting upon pyrolysis to ceramic. The oxygen treatment led to the presence of up to 20% SiO.sub.2 in the final ceramic. This process is currently used by Nippon Carbon Company to produce Nicalon.RTM. fibers. The polycarbosilane used in this process has a molecular weight of .apprxeq.1,500 and is marketed by Dow Corning Corporation as a ceramic precursor. An oxidative treatment is required and an 80% yield of ceramic (containing 20% SiO.sub.2) is obtained. Pyrolysis of unoxidized material results in a lower (.apprxeq.60%) ceramic yield. The presence of large amounts of SiO.sub.2 in the ceramic is known to have adverse effects on the mechanical properties at high temperatures. [See Hasegawa et al, J. Mat. Sci., Vol. 21, p. 4352 (1986)]. One review of the properties of Nicalon.RTM. fibers states that "[i]t seems clear that a fiber having a very low oxygen content would be desirable and this could only be achieved by changing the process of conversion to the precursor into the finished fiber." [Bunsell et al, Composites Science and Technology, Vol. 27, p. 157 (1986).] This has been accomplished to some degree by an electron beam cross-linking process; however, this is an expensive and complicated procedure [Okamura, Composites, Vol. 18, No. 2 (1987)]. Even though these fibers have serious drawbacks, they are considered to be state of the art and are currently enjoying widespread use in the area of composite materials.
It is an object of the present invention to provide novel SiC fibers having an extremely low oxygen content and superior high temperature mechanical properties to those produced according to present day methods.
It is another object of the invention to provide novel methods for the preparation of the above-described SiC fibers as well as intermediates therefor.